A particulate tape is a ribbon or sheet of a particulate material in which the particles are held together, for example, by a polymer binder or matrix.
Particulate tapes are used in the manufacture of a variety of products. In the simplest case, the particulate tape itself can be the product. An example of this is magnetic recording tape which is used in tape form in video cassettes or cut into disks for use as media in computer floppy disks. This tape is made up of magnetic particles in a polymer matrix with a polymer film backing for strength.
Another use of particulate tapes is as an intermediate in the manufacture of more complex parts, particularly ones having a laminate structure. This can include structural components such as shells or panels made by tape lay up, or tubular structures made by tape winding. These structures can be used “as is,” however, it is usually desired to form a dense part having only the properties of the material that is contained in particulate form. In this case the laminated part will go through a process whereby the binder is removed, followed by a heat treatment to sinter the particles, thereby forming a solid, dense structure.
One of the more complex and demanding uses of particulate tapes is as the layers in the manufacture of multi-layer electronic devices and circuits. Multi-layer fabrication can be used to make single element electronic devices such as inductors, resistors, capacitors, transformers and transducers, for example. Multi-layer fabrication can also be used to make parts that include more than one device and more than one kind of device, along with conductor paths to connect these devices. The tapes used to form these parts must incorporate patterns of different particulate materials both in the plane of the tape and through the thickness of the tape to connect adjacent layers.
The current method of manufacture of these multi-layer electronic components begins with making a generally uniform, single component particulate tape. There are several possible methods for making these tapes including: waterfall casting, dip coating, and spraying, for example. However, the most common process is doctor blade tape casting, which is now described. An example doctor blade apparatus for casting a tape is shown in FIG. 1.
The first step in casting a particulate tape is forming a slip (or slurry) which is a suspension of the powder (particulate) material of which the tape is to be made. The slurry typically includes the particles, a solvent to make the slurry fluid, an organic dispersant that coats the particles so individual particles can slide over and past each other in the slip, and a binder to give the tape the mechanical properties necessary for handling after drying. The solvent dissolves the binder and has sufficient volume and sufficiently low viscosity to allow the slurry to flow. As the slurry dries, the binder coats the solid particles and bind them together.
To cast a tape, the slip is placed into a hopper having an open bottom end placed over a moving carrier. At the trailing edge of the bottom of the hopper, there is a small gap parallel to and just above the surface of the carrier. The height of this gap can be adjusted by moving a doctor blade, located at the top of the gap, up and down. As the carrier passes under the slip hopper, it draws a layer of the slip with it. The thickness of the layer of slip on the surface of the carrier is determined by the height of the doctor blade and the flow characteristics of the slip. The result is a thin, uniform layer of slip on the surface of the tape carrier.
Once the slip is cast onto the carrier, it is dried to form a tape that can be handled in the subsequent processing steps. As the layer of cast slip is conveyed on the carrier away from the hopper, the solvent evaporates. This evaporation can be controlled by passing the tape through an elongated, temperature and atmosphere-controlled drying chamber. During drying, the particles are typically pulled together to final density by capillary forces.
To create electronic devices from the cast tape, additional components need to be added to the cast tape. For example, for multi-layer capacitors, particles of a conductive material need to be printed onto the surface of a tape to form inner metal electrodes in the final sintered device.
If internal interconnections between layers are needed in the final device, vias must first be added. Vias are formed by mechanically punching holes in the tape and refilling the holes with a particulate paste. This paste contains a powder which, when sintered, will electrically or magnetically interconnect two layers.
After the vias have been punched and filled, patterns of conductors and other materials are printed on the surface of the tape. These additional features are printed onto the surface of the tape in the form of a thick film ink or paste. While there are a variety of printing processes that can be used, the most commonly used process is screen printing. In this process, a thin metal screen is pressed against the tape. The screen has a pattern of open and filled holes which correspond to the pattern of ink that is to be printed on the tape. The ink is placed on top of the screen, and a rubber squeegee is passed over the top of the screen, forcing the ink into the open holes in the screen. The ink sticks to the surface of the tape, and when the screen is pulled off, a pattern of ink is left on the surface of the tape. The ink should flow somewhat so as to fill in the gaps between adjacent holes, but should not flow so much as to destroy the edge definition of the pattern being printed.
Once these multi-component tapes have been formed they are stacked, laminated, and cut; the binder is removed; and the part is sintered. The stacking process involves careful alignment of the tapes to insure that the features printed on the tapes align in the vertical direction. Lamination is a process of pressing the tapes together at a slightly elevated temperature to weld them into a single body that will not delaminate during sintering. The elevated temperature plastically deforms the binder, which allows the particles to move and create a more intimate contact between the layers. In cases where the tape is too thin to be handled independently, it is left on a carrier until it is laminated to a stack, after which the carrier is peeled off. The stack is built up by laminating one layer at a time. The stack is then cut into individual parts, the polymer binder is removed—usually by heat treatment in a controlled atmosphere—and the parts are sintered.
Finally, after the part is sintered, external interconnections between layers can be added, if necessary. The conductor patterns to be interconnected are designed to intersect the cut edge of the part at a common point. All of the conductors that are exposed on the side of the part at that point can then be interconnected by applying a vertical strip of a conductive material, known as an edge connector, to the side of the part.
The simplest example of using an edge connector is the multi-layer capacitor shown in FIG. 2. In this device, it is desired to connect alternating layers of conductor 27 (separated by intervening layers 28) to the opposite poles of the capacitor. To do this the capacitor is designed as a multi-layer rectangular block 30 with even numbered conductor layers intersecting one end of the block and odd layers intersecting the opposite end (FIG. 2B). After the part is sintered, it is tumbled in an abrasive powder to polish the end of the block, assuring a good contact surface on conductor layers at each end. A conductive ink, frequently referred to as termination compound, is then applied to each end of the block 32. This ink is then dried and sintered in a second heat treatment to form the electrical interconnect between these alternating layers.
In view of the foregoing, a need has been recognized in connection with the provision of single and multiple component particulate tapes that can be produced with one or more of the following characteristics: improved uniformity of particle packing, relatively uniform thickness of multiple component tapes, reduced thickness of the complete tape as well as of individual component layers within a tape, improved lateral resolution of component patterns on and in the tape, and the forming of patterns of components that extend through the thickness of a multi-component tape.